The present invention relates to a method and an apparatus for controlling a continuously variable transmission, and more particularly to a method and an apparatus for controlling a lock-up clutch of a hydrodynamic device of a continuously variable transmission of an automotive vehicle.
The term "hydrodynamic device" is used herein to refer to a fluid coupling and a torque converter.
In a method for controlling a continuously variable V-belt transmission which was previously proposed in pending Japanese Patent Application No. 56-44749 filed Mar. 28, 1981 which corresponds to copending U.S. patent application Ser. No. 362,489 filed Mar. 26, 1982 and commonly assigned herewith, a lock-up clutch was engaged at vehicle speeds above a relatively low vehicle speed value for better fuel economy by eliminating a power loss due to slip in the hydrodynamic device, and the engagement of the lock-up clutch was released only in a low vehicle speed operating range in order to prevent the occurrence of unpleasant vibrations at low speeds, and to secure a traction force great enough for moving the vehicle from standstill.
However, with the above method, the acceleration performance was not satisfactory because the hydrodynamic device was maintained in its engaged condition even when the transmission was to shift down to a large reduction ratio to meet a rapid acceleration demand. The engine could not increase its speed promptly for greater torque.